The present application is based on Japanese Patent Applications No.2002-128790 and No.2002-276184, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The invention relates to a light emitting diode which is usable, for example, in light sources for photosensors, printers, and backlights of various displays and the like. More particularly, the invention is concerned with a light emitting diode including a light emitting element and a phosphor material, in which light of a desired uniform color can be radiated from the light emitting diode as viewed from an emission observing surface of the light emitting diode.
In this specification, an LED chip per se is referred to as xe2x80x9clight emitting element,xe2x80x9d and the whole system including an LED chip-mounted package resin or lens system or other optical system is referred to as xe2x80x9clight emitting diode.xe2x80x9d
2. Related Art
A conventional light emitting diode, which uses a light emitting element and a phosphor to emit light of a desired color, is schematically shown in FIG. 1. This light emitting diode includes a pair of lead frames 102, 103. The lead frame 102 has a concave part in its leading end. A light emitting element 101 is mounted on the bottom of the concave part. The light emitting element 101 is covered with a phosphor-mixed resin 104. Further, the space within of the concave part is filled with the phosphor-mixed resin 104. The phosphor-mixed resin 104 is a resin, such as a transparent epoxy resin, into which a phosphor has been mixed. This phosphor absorbs light emitted from the light emitting element 101, performs wavelength conversion of the absorbed light, and gives off light with a wavelength different from the absorbed light. The whole assembly has been sealed with a sealing resin 105.
In the light emitting diode having the above construction, assuming that, for example, the light emitting element 101 is a blue light emitting element and the phosphor is one which absorbs blue light emitted from the blue light emitting element, performs wavelength conversion of the blue light and gives off yellow light, the blue light emitted from the blue light emitting element is mixed with the yellow light given off from the phosphor. Therefore, theoretically, white light can be radiated to the outside of the light emitting diode as viewed from an emission observing surface of the light emitting diode.
This light emitting diode, however, suffers from a problem that, as viewed from the emission observing surface of the light emitting diode, color of light radiated from the light emitting diode varies from position to position of the emission observing surface of the light emitting diode. The above unfavorable phenomenon will be hereinafter referred to as xe2x80x9clack of uniformity of color of radiated light.xe2x80x9d This problem is attributable to the following fact. A part of light is radiated from the light emitting element 101 in a direction just above the light emitting element 101. Another part of light is radiated obliquely from the top surface or side surface of the light emitting element 101. Further, after emission from the light emitting element 101, a part of light is reflected from the inner wall of the concave part. Therefore, the amount of the phosphor, in the phosphor-mixed resin 104, through which the light is passed in a period between the emission of the light from the light emitting element and the emergence of the light from the light emitting diode varies depending upon optical paths through which the light is passed and radiated to the outside of the light emitting diode. This variation in the amount of phosphor will be hereinafter often referred to as xe2x80x9ca difference in the amount of light-permeated phosphor among the optical paths.xe2x80x9d In this case, as viewed from the emission observing surface side of the light emitting diode, the emission observing surface in its portion just above the light emitting element 101 is perceived as emitting white light, and a portion around that portion just above the light emitting element 101 is perceived as emitting yellow light. Thus, the light radiated from the light emitting diode lacks in uniformity of color.
In particular, in recent years, there is an ever-increasing demand for a reduction in size and a reduction in thickness of the light emitting diode. To meet this demand, for example, shell-shaped or SMD (surface mounted device) type light emitting diodes have been desired in the art. In this case, in order to realize the small and thin light emitting diode, the concentration of the phosphor in the phosphor-mixed resin, which covers the light emitting element and fills up the space within the concave part, should be enhanced. The enhancement in the concentration of the phosphor in the phosphor-mixed resin can certainly meet the demand for a reduction in size and a reduction in thickness of the light emitting diode. In this case, however, the difference in the amount of light-permeated phosphor among the optical paths is further increased. Therefore, the lack of uniformity of color of radiated light becomes more significant.
Japanese Patent No. 3065263 proposes a light emitting diode which reduces the lack of uniformity of color of radiated light. This light emitting diode is shown in FIG. 2. As shown in FIG. 2, the light emitting diode includes a reflecting member 127 having a concave part. A light emitting element 123 is mounted on the bottom of the concave part. The light emitting element 123 is provided with a pair of electrodes (not shown). These electrodes are bonded respectively to external electrodes 124 through electrically connecting members 126. The light emitting element 123 and the electrically connecting members 126 are covered with and are embedded in a first resin 121. The first resin 121 is in a concave spherical surface as viewed from an emission observing surface of the light emitting diode. The remaining space, on the first resin 121, of the concave part is filled with a second resin 122 which is a phosphor-mixed resin. The claimed advantage of this construction is to substantially reduce the difference in the amount of light-permeated phosphor among the optical paths, and thus to reduce the lack of uniformity of color of light radiated from the light emitting diode.
In the above light emitting diode, however, the thickness of the phosphor-containing second resin 122 is the largest in a portion just above the light emitting element 123 and gradually decreases toward the side wall of the concave part. A part of light emitted from the light emitting element 123 is radiated in a direction just above the light emitting element 123 to the outside of the light emitting diode through the second resin 122 in its large thickness portion. On the other hand, another part of light emitted from the light emitting element 123 advances toward the inner wall of the reflecting member 127, is reflected from the side wall of the concave part, and advances toward the outside of the light emitting diode through the second resin 122 in its small thickness portion. Therefore, regarding the emitted light passed through an optical path including the small-thickness portion of the second resin 122, the amount of light-permeated phosphor is smaller than that in the emitted light passed through the other optical paths. That is, a difference in the amount of light-permeated phosphor among optical paths exists, and, thus, the light radiated from the light emitting diode lacks in uniformity of color. In other words, also in the above light emitting diode, light radiated from the light emitting diode disadvantageously lacks in uniformity of color as viewed from the emission observing surface of the light emitting diode.
Further, in the above light emitting diode, light emitted from the light emitting element 123 is directly incident in a high-optical density state to the phosphor mixed second resin 122. According to experiments conducted by the present inventors, for many phosphor materials, the emission efficiency is likely to decrease with the increasing the quantity of light absorbed (optical density). As a result, it was confirmed that, even when the brightness of light emitted from the light emitting element has been increased, the emission efficiency of the phosphor material is saturated at a predetermined value and cannot be enhanced. For this reason, in the above conventional light emitting diode, the radiation of light having a predetermined color through wavelength conversion using a phosphor material with high emission efficiency could not have been realized without difficulties.
Under the above circumstances, the invention has been made, and it is an object of the invention to provide a light emitting diode which can eliminate the problem of lack of uniformity of color of radiated light involved in the prior art and can radiate light having a desired uniform color as viewed from an emission observing surface of the light emitting diode.
It is another object of the invention to provide a light emitting diode which can realize emission of light from a phosphor material with high efficiency and can realize high-brightness light of a predetermined color.
According to the first feature of the invention, a light emitting diode comprises: a reflecting mirror having a concave part; a light emitting element disposed within the concave part of the reflecting mirror; a light transparent layer formed of a first light transparent material which has been filled into a space within the concave part of the reflecting mirror so as to cover the light emitting element; and a fluorescence emitting layer provided on the light transparent layer on its emission observing surface side, said fluorescence emitting layer being formed of a second light transparent material which is different from the first light transparent material, said second transparent material containing a phosphor material that absorbs light which has been passed through the light transparent layer and emits light with a wavelength different from the absorbed light.
According to the above construction, since the first light transparent material is different from the second light transparent material, light is irregularly reflected from the interface of the first light transparent material and the second light transparent material, because the refractive index of the first light transparent material is different from that of the second transparent material. As a result, the probability of incidence of light having high optical density, which has been emitted from the light emitting element, directly to the phosphor material contained in the fluorescence emitting layer is lowered, and, instead, the probability of incidence of this light in a dispersed state to the phosphor material contained in the fluorescence emitting layer is increased. Therefore, the light can be introduced in various directions from the light transparent layer into the fluorescence emitting layer to enhance the emission efficiency of the whole fluorescence emitting layer, and light having a uniform color can be radiated from the light emitting diode as viewed from the emission observing surface of the light emitting diode.
According to the second feature of the invention, a light emitting diode comprises: a reflecting mirror having a concave part; a light emitting element disposed within the concave part of the reflecting mirror; a first layer formed of a phosphor material-containing light transparent material which has been filled into a space within the concave part of the reflecting mirror to the level of the upper surface of the light emitting element disposed within the concave part of the reflecting mirror; and a second layer formed of a phosphor material-containing light transparent material which is provided on the first layer on its emission observing surface side, said phosphor material contained in the first and second layers being capable of absorbing light emitted from the light emitting element and emitting light with a wavelength different from the absorbed light, the concentration of the phosphor in the second layer being higher than the concentration of the phosphor in the first layer.
According to the above construction, lights emitted in various directions from the light emitting element are substantially equal to one another in the amount of phosphor through which the lights are passed in a period between the emission from the light emitting element and the radiation from the light emitting diode after passage through the first and the second layers. Therefore, during passage through the first and second layers, light given off from the phosphor after wavelength conversion can be uniformly mixed with light not passed through the phosphor, and, thus, light having a desired color can be uniformly radiated from the light emitting diode as viewed from an emission observing surface of the light emitting diode.
According to the third feature of the invention, a light emitting diode comprises: a reflecting mirror having a concave part; a light emitting element disposed within the concave part of the reflecting mirror; and a phosphor material-containing light transparent material layer which has been filled into a space within the concave part of the reflecting mirror so as to cover the light emitting element, said phosphor material being capable of absorbing light emitted from the light emitting element and emitting light with a wavelength different from the absorbed light, said phosphor material having been distributed in said light transparent material layer so that values obtained by multiplying the length of optical paths, through which lights emitted from the light emitting element are passed and reach the upper surface of the light transparent material layer, by the concentration of the phosphor material are substantially equal to one another.
According to the above construction, lights emitted in various directions from the light emitting element are substantially equal to one another in the amount of phosphor through which the lights are passed in a period between the emission from the light emitting element and the radiation from the light emitting diode after passage through the light transparent material. Therefore, during passage through the light transparent material, light given off from the phosphor after wavelength conversion can be uniformly mixed with light not passed through the phosphor, and, thus, light having a desired color can be uniformly radiated from the light emitting diode as viewed from an emission observing surface of the light emitting diode.
According to the fourth feature of the invention, a light emitting diode comprises: a reflecting mirror having a concave part; a light emitting element disposed within the concave part of the reflecting mirror; a transparent light guiding/scattering layer which has been filled into a space within the concave part of the reflecting mirror so as to cover the light emitting element, said transparent light guiding/scattering layer being capable of scattering in various directions light emitted from the light emitting element; and a phosphor material-containing transparent fluorescence emitting layer which is provided on the light guiding/scattering layer in its emission observing surface side, said phosphor material being capable of absorbing light, which has been emitted from the light emitting element and passed through the light guiding/scattering layer, and capable of emitting light with a wavelength different from the absorbed light.
According to the above construction, the probability of incidence of light having high optical density, which has been emitted from the light emitting element, directly to the phosphor material contained in the fluorescence emitting layer is lowered, and, instead, the probability of incidence of this light in a dispersed state to the phosphor material contained in the fluorescence emitting layer is increased. Therefore, the light can be introduced in various directions from the light guiding/scattering layer into the fluorescence emitting layer to enhance the emission efficiency of the whole fluorescence emitting layer, and light having a desired uniform color can be radiated from the light emitting diode as viewed from the emission observing surface of the light emitting diode.